They are medium to large birds with strong feet and
bills, rictal bristles and a single
moult each year (most passerines moult twice).

Corvids are found worldwide except for the tip of
South America and the polar ice caps (Clayton and Emery
2005). Recently the Corvus genus has re-entered
Australia, resulting in five new species and one new
subspecies (see
crows). The majority of the species are found in
tropical South and
Central America, southern Asia and Eurasia, with fewer than
10 species each in Africa, Australasia and North America

Systematics, taxonomy and evolution

The earliest corvid fossils date to the mid-Miocene
(about 17 MYA) [2]. The genus Corvus, including the crows
and ravens, makes up over a third of the entire family. The
name Corvus was given to these birds is onomatopoetic, from
their raucous “croaking” calls [3]. Corvids are derived from
Australasian ancestors and from there, spread throughout the
world. Other lineages derived from these ancestors evolved
into ecologically diverse, but often Australasian groups.
Over the years there has been much disagreement on the exact
evolutionary relationships of the corvid family and their
relatives. Sibley and Ahlquist have united the corvids with other
taxa in the
Corvida, but current research favors the theory that
this grouping is partly artificial.

Clarification of the interrelationships of the corvids
has been researched by Ericson et al. (2005), based
on comparison of several DNA sequences. The Crested Jay (Platylophus
galericulatus) is traditionally included in the Corvidae,
but seems not to be a member of this family. Likewise, the
Hume's Ground "Jay" (Pseudopodoces humilis) is in fact a
member of the family (Paridae) (titmice). The jays and
magpies do not constitute monophyletic lineages, but rather
seem to split up in a American and Old World, and a
Holarctic and a Oriental lineage, respectively, which
are not closely related inter se. The position of the
Azure-winged Magpie, which has always been a major
enigma, is even more unclear than it was before.

In addition, there are numerous fossil species of extant
genera (mainly European Corvus). See the genus
accounts for more.

Typical size and appearance

Corvids have feathered rounded nostrils, strong tails and
wings and similar sexes. Many corvids of temperate zones are
mainly black or blue; however, some are pied black and
white, some have a blue-purple iridescence and many tropical
species are highly coloured. Corvids have strong, stout
bills, large wingspans and are between 23 and 71 cm long.
[4]

Members of the genus Corvus are the largest
members of the passerine order reaching 50-71 cm (20-27
inches). Species can be identified based on size, shape and
geography; however, some, especially the Australian crows,
are best identified by their raucous calls.
[5]

Social interaction

Some corvids have strong organization and community
groups. Jackdaws, for example, have a strong social
hierarchy, and are facultatively colonial during breeding
(Verhulst and Salomons 2004). Providing mutual aid has also
been recorded within many of the corvid species.
[6]

Young corvids have been known to play and take part in
elaborate social games. The games resemble “king of the
mountain” and “follow the leader” along with games that
manipulate, pass and balance sticks. Corvids also take part
in other activities, such as sliding down smooth surfaces,
and these games are understood to play a large role in the
adaptive and survival value of the birds (Gill 2003).

Some corvids can be aggressive birds.
Blue Jays, for example, are well known to attack
anything that threatens their nest. Crows have been known to
attack dogs, cats, ravens, and birds of prey. Most of the
time these assaults take place as a distraction long enough
to allow the crow to steal food.[7]

Food and foraging habits

The natural diet of many corvid species is omnivorous,
consisting of invertebrates, nestlings, small mammals,
berries, fruits, seeds, and carrion. However, some corvids,
especially the crows, have adapted well to human conditions
and have come to rely on anthropogenic foods. In a US study
of American Crows, Common Ravens and Steller's Jays around campgrounds and human settlements,
the crows appeared to have the most diverse diet of all,
taking anthropogenic foods such as bread, spaghetti, fried
potatoes, dog food, sandwiches, and livestock feed. The
increase in available anthropogenic food sources is
contributing to population increase in some corvid species.
(Marzluff and Neatherlin 2006).

Some corvids are predators of other birds. During the
wintering months, corvids typically form foraging flocks
[8]. However, some crows also eat many agricultural
pests including cutworms, wireworms, grasshoppers and
harmful weeds
[9]. Some corvid will eat carrion, and since they lack a
specialized beak for tearing into flesh, they must wait
until animals are opened by other predators or as roadkill.

Migration

Corvids occur in most climatic zones. Most are sedentary
and do not
migrate significantly. However, during a shortage of
food, eruptive migration can occur
[10]. When species are migratory, they will form large
flocks in the fall (around August) and travel south
[11].

Reproduction

Some corvids are well known communal roosters. Some
groups of roosting corvids have been as large as 2,000 birds
(Everding and Jones 2006). The partner bond in corvids is
extremely strong and even lifelong in some species. This
monogamous lifestyle, however, can still contain extra pair
copulations. Males and females build large nests together in
trees or on ledges. The male will also feed the female
during incubation
[12]. The nests are constructed of a mass of bulky twigs
lined with grass and bark. Corvids can lay between 3 and 10
eggs, typically ranging between 4 and 7. The eggs are
usually greenish in colour with brown blotches. Once
hatched, the young remain in the nests for up to 6–10 weeks
depending on the species. As expected, corvids provide
biparental care.

Sexual selection is also quite complex in the Corvidae
family. Young corvid members undergo a series of tests,
including aerobatic feats, before being accepted as a mate
by the opposite sex
[13].

Unlike most other species, corvid fitness and
reproduction, especially with the crows, has increased due
to human development. The survival and reproductive success
of crows and ravens, according to Marzluff and Neatherlin’s
2006 study, was positively associated with their intimacy of
human populations.

Human development provides additional resources by
clearing land, creating shrublands rich in berries and
insects. When the cleared land naturally replenishes, the
young dense trees are used by jays and crows for nesting
sites. Ravens typically use larger trees in denser forests
(Marzluff and Neatherlin 2006).

One reason for the success of crows, compared to ravens,
is their ability to overlap breeding territory. During
breeding season, crows were shown to overlap breeding
territory six times the overlap of ravens. This invasion of
breeding ranges allowed a related increase in local density
(Marzluff and Neatherlin 2006). In the US the American Crow
population has definitely grown over the years. It is
possible, that the American Crow, due to humans increasing
suitable habitat, will drive out the Northwestern and Fish
Crows (Marzluff and Angell 2005).

Jackdaws can breed in buildings or in rabbit warrens
(Verhulst and Salomons 2004). White-throated Magpie-jays are
cooperatively breeding corvids where the helpers are mostly
female. Cooperative breeding takes place when additional
adults help raise the nestlings. These adults are often
called “helpers” and in most cooperatively breeding birds the males take on
the “helper” role while females join other groups (Berg
2005).

Nest predation

Since crows do not seem to mind human development, it was
suggested that the crow population increase would cause
increased rates of nest predation. However, the Steller’s
Jays, which were successful independent of human
development, were the more frequent nest predator.
Therefore, the human relationship with crows and ravens did
not increase nest predation since jays accounted for the
most nest predation by corvids (Marzluff and Neatherlin
2006).

Myths

Since some corvids, especially in the temperate Northern
Hemisphere have black feathers and eat carrion, humans have
long associated members of Corvidae with death and extreme
injustice (Marzluff and Angell 2005). Throughout history,
corvids have been perceived as dark messengers, bearing ill
will and other demonic associations. This dark connection is
reflected by the literary terms coined to describe groups of
crows (a murder), ravens (unkindness, constable or
conspiracy), and jays (scold).
[14].

Despite the well-known demonic association, folklore also
represents corvids as wise animals. Native Americans
believed that a raven created the earth, the Norse god Odin
constantly sought the advice of ravens, and even Aesop
featured corvids as smart heroes in many fables (Clayton and
Emery 2005). According to native cultures, despite being a
trickster spirit, ravens were popular on totems, were
credited with creating man and were responsible for placing
the Sun in the sky. In western literature, popularized by
E.A. Poe, the
Common Raven was a symbol of darkness, depression and
death. However, in mediaeval times the raven stood for
virility. Legends report that a raven’s favourite food is
dead animals, and that they sometimes hunt with wolves
[15]. For more myths and legends see
crow and
raven pages.

Corvid intelligence

Corvids contain the largest brain, relative to their body
size, of any bird. Based on a brain-to-body ratio, the
corvid brain equals the size of a chimpanzee, is almost the
same as a dolphin, and is only slightly lower than a human
[16]. Their intelligence is evident due to the long
developmental period of the young. By remaining with the
parents, the young have more opportunities to learn
necessary skills. Since most corvids are cooperative
breeders, their young can learn from different members of
the group (Clayton and Emery 2005). Some naturalists argue
that the Corvidae family contains intelligence superior to
that of all other bird species
[17]. When compared to other carnivorous mammals
(specifically dogs and cats) in one laboratory test, corvid
birds outshone their components, demonstrating operational
abilities almost as excellent as monkeys (Krushinskii et al
1979). Dr. Louis Lefebvre’s avian IQ test declared Corvidae
the most intelligent bird based on the scale
[18].

The corvid ingenuity is represented through their feeding
skills, memorization abilities, use of tools, and group
behaviour. Living in large social groups has long been
connected with high cognitive ability. To live in a large
group, a member must be able to recognize individuals and
track the social position and foraging of other members over
time. Members must also be able to distinguish between sex,
age, reproductive status, dominance and be able to update
the information constantly. Therefore, social complexity
directly corresponds to high cognition (Bond et al 2003).

There are also specific examples of corvid cleverness.
One crow was documented to crack nuts by placing them on a
crosswalk, letting the passing cars crack the shell, waiting
for the light to turn red, and then safely retrieving the
contents. A group of crows in England took turns lifting
garbage bin lids while their companions collected food.
Members of the corvid family have been known to watch other
birds, remember where they hide their food, then return once
the owner leaves. Corvids also move their food around
between hiding places to avoid thievery, but only if they
have previously been thieves themselves. The ability to hide
food requires highly accurate spatial memories. Corvids have
been recorded to recall their food’s hiding place up to nine
months later. It is suggested that vertical landmarks (like
trees) are used to remember locations. There has also been
evidence that western scrub-jays, who store perishable
foods, not only remember where they stored their food, but
for how long. This is compared to human episodic memory,
which was previously thought unique to humans (Clayton and
Emery 2005).

Looking at the act of thievery in the corvid family, it
has been suggested that birds will take their experience as
a thief and use it to predict other bird actions of
thievery. This explains why, if a corvid has committed
thievery, they will take extra precautions (such as moving
hiding places) to avoid being a future victim. Being able to
predict others behaviour based on your own experiences is
another trait previously thought unique to humans.
Laboratory experiments have confirmed that specifically
crows, can sometimes use a past experience to approach a new
obstacle (Clayton and Emery 2005).

Caledonian Crows have been observed to make tools of
twigs trimmed into hooks. They then use to hooks to pull
insect larvae from tree holes. Caledonian crows are not the
only corvids to use tools, and diversity in tool design
among corvids suggest cultural variation. Again, apes are
the only other animals known to use tools in such a fashion
(Clayton and Emery 2005). Nutcrackers and jackdaws were
compared in a 2002 study based on geometric rule learning.
The corvids, along with a pigeon, had to locate a target
between two landmarks, during which distances and landmarks
were altered. The nutcrackers resulted in searching more
accurately than the jackdaw and pigeon (Jones et al 2002).

A very popular crow scare tactic in the agricultural
business is the scarecrow. However, due to the corvid’s
quick wit, scarecrows are soon ignored and used as perches.
Despite farmers efforts to rid themselves of corvid pests,
their attempts have only expanded corvid territories and
strengthened their numbers
[19]. Recent taxonomy places corvids, based on their
evolutionary progress, in the middle of the passerines,
despite efforts to promote them to the most advanced of the
birds
[20].

Threatened species

Despite the fact that most corvids are not threatened
(but are most likely secured by human interaction) a few
species are in danger. For example, the destruction of the
Southeast Asian rainforests is endangering mixed-species
feeding flocks with members from the family Corvidae (Lee et
al 2005). Also, since scrub is an endangered ecosystem, the
Florida Scrub-jays are threatened with extinction
(Breiniger, et al 2006).